The next steps for Airbus' big bet on open rotor engines
aerospaceamerica.aiaa.org66 points by CGMthrowaway 9 hours ago
66 points by CGMthrowaway 9 hours ago
It's part of the tradeoff between momentum and energy that you should aim to move as high of a mass of air at as low of a speed as possible for efficiency.
When you put energy into a mass of air you impart energy of 1/2 MV^2, the kinetic energy equation, which you can think of as the energy you're leaving in the air as it's accelerated to a given velocity on exhaust from the engine. The V^2 part is a killer. This does not translate directly into momentum at all and the most energy efficient way to gain momentum is with a large mass that's accelerated to a low velocity. You can actually see this with the wings which keep the plane itself up. The wings impart enough momentum to hold the weight of the aircraft up by moving a lot of air at relatively low velocity which sacrifices very little energy for the upwards momentum gained.
So engines in aircraft have been getting bigger and bigger as well as slower and slower. It's basic physics, aiming to move as high of a mass at as low of a practical velocity as possible. The 737 max issues were an example of adding giant engines to an airframe not originally built for them due to the drive to move as much air at as low of a velocity as possible while still keeping the plane moving forwards. Passenger aircraft have been getting slower over the years, the 747 was faster than the newer 787's because we're looking for efficiency above all else these days. Going open bladed makes a lot of sense as we go further down this path.
This isn't true though, the 747's cruise speed is the same as the 787's at 0.85 mach. The 747 has a slightly faster max speed but that's not relevant for actual travel. The 777 has a slower max speed and cruise speed than the 787 despite being older. I don't think you can realistically draw a correlation between older/newer being faster or slower on wide body aircraft.
IDK if "bigger and slower aircraft" is what he meant, but rather "bigger and slower engines." Jets cruise @ mach .85 because that's the economic optimum set by wave drag, compressibility and passenger time costs. Hasn't changed in 50 years.
The relevant metrics are amount of air moved and speed the air is accelerated to, aka efficiency gains from propulsive efficiency- e.g. increasing bypass ratio, larger fan diameter, lower jet exhaust velocity
Sounds like a helicopter is not very efficient?
Less efficient than an aircrafts wings over a long distance but very efficient for an aircraft with engines pointing straight down.
The blades are massive, push a lot of air relatively slowing compared to smaller engines. There's a reason most planes will stall when pointing straight up, despite in theory having more power to weight. Their prop efficiency is worse than a helicopters rotors.
I'm curious about using a hybrid system where you have multiple electric fans. For instance 2 turbines and 4 fans. Advantage is smaller diameter for the same mass flow. And more redundancy. A negative is the weight of the electric motors and generators. If you added a battery you have some other advantages. Less pucker when you lose an engine. And better throttle response.
Another advantage is you can place the fans all along the wing getting you better stall resistance as the flow doesn't detach as easily. There's already a prototype of a hybrid plane that does this:
You can go one further and just mount a squirrel cage fan in place of or on the front or top of the wing. https://en.wikipedia.org/wiki/FanWing
Or go further and use rotating drums: https://en.wikipedia.org/wiki/Flettner_airplane
Or you can use a horizontal-axis style helicopter rotor with variable pitch, and it gets you omnidirectional thrust (VTOL) https://en.wikipedia.org/wiki/Cyclogyro
There are a lot of interesting possible alternate histories (only requiring a few tweaks to physics) where fixed wings never really work and horizontal rotorcraft dominate, especially in a world where lighter-than-air craft are common - something like a hybrid between a zeppelin and a paddleboat.
Like a lot of people I think I hold a mental image of "jet" which is actually not helpful for a modern engine. All modern jets seem to have this massive rotational component, the turbine, and the fan outside the turbine chamber. so does a turboprop. And the basic propeller before that. Oh, the "fan" has more blades. Pshaw! a spitfire went from 3 blades to 5 across it's lifetime. post-spitfire engines had contra rotating props with many more blades. It can't just be about the NUMBER of blades can it?
So, there is the turbine. Is that directly coupled to the "fan" bit? If not, it's probably a turboprop, but even then I am unsure all visible fans on modern jets on the spool couple directly to the turbine.
The "jet" part is the combustion chamber. Everything else, you might as well consider turbines and propellers as "the same kind of thing" but then you're in a pub arguing which details make one a prop and the other a fan.
If you like Roger ramjet you're in the other kind of Jet: the one which is more like a rocket. Also, if you work in government service how are you passing the drug test with those proton energy pills?
Frank Whittle's biography is a great read. He had some hair raising moments. things OSHA would not be happy about.
Most modern engines have two turbines. A low pressure turbine and a high pressure turbine. The low pressure turbine is connected directly to the fan.
> I am unsure all visible fans on modern jets on the spool couple directly to the turbine
They exist now [1][2]. The general term is geared turbofans.
If you want to mentally unfuck it a bit, the major variables are: combustion type (internal or turbine), gearing or not, ducting or not and bypass ratio. (Compression ratio and number of blades can come too.)
When one of these changes substantially, you get a change in engine type. When it changes a little bit, you get a blur.
[1] https://en.wikipedia.org/wiki/Pratt_%26_Whitney_PW1000G
[2] https://en.wikipedia.org/wiki/Rolls-Royce_Trent#UltraFan
Which bio?
This one is the one I have a hard copy of from 1954:
"jet" -the story of a pioneer by Sir Frank Whittle
https://www.amazon.com.au/Jet-Story-Pioneer-Pioneers-Aviatio...
Isn't this like turboprops (already very efficient) with bigger propellors? I couldn't tell from the article, but quite possibly missed something.
This website has some nice explanations and GIFs: https://s2.smu.edu/propulsion/Pages/variations.htm
So the "open rotor" engine is actually a propfan engine?
Essentially yes, different engine companies have used different nomenclature over time. It seems that the "open rotor" terminology is being used to emphasize the improvements which have been made to blade design, noise, and general efficiency.
I think it is the stators fixed to the engine nacelle, judging by the article image.
> Airbus is also assessing shielding the area of the fuselage closest to the engines to minimize the risk of a blade off — one or more composite blades breaking, which could dent or puncture the fuselage and, in the worst-case scenario, strike a passenger.
sightly terrifying
The cowling of the current turbines serves the same purpose, but needs to cover 360 degrees of rotation, so it's heavier and draggier. The blades have a bit more angular momentum in the propfan than in a high bypass turbofan, but there's fewer of them.
Instead of reinforcing the fuselage, I wonder if just having a 1/4 nacelle that shields the passenger side would work.
The impact area of the fuselage looks much larger than an unrolled cowling, and thus significantly heavier to reinforce. The smaller cowling will save drag through.
>The cowling of the current turbines serves the same purpose, but needs to cover 360 degrees of rotation
this doesn't make sense. if you are not worried about fan blades flying off in directions other than the fuselage, why cover 360 degrees? (and if you are worried 360, then why open rotor?)
The cowling is its own structural support, so needs to be strong all around, otherwise it would fail on the other side and you'd get blade+cowling approaching the fuselage at high velocity.
Reminded me of this: https://youtu.be/j973645y5AA?si=QJrNJe0gT-zwpElD
Seems like quite an engineering challenge with this new design...
I had a sharp intake of breath after reading this and then clicking through to see the header image of the article.
High bypass turbo fans do this as well, it's just in the fan/engine housing, not the fuselage.
Not clear to me from the article - what's the different between an 'open rotor' engine and a turboprop (https://en.wikipedia.org/wiki/Turboprop)? At face value, both seem to be jet engines with propellers used on single-aisle planes?
There is still a jet stream coming out of the engine propelling the aircraft. Unlike a turboprop where only the propellor generates thrust.
> Unlike a turboprop where only the propellor generates thrust
Each of turbojets, turboprops and turbofans generate thrust with exhaust.
Interesting - I'm curious to learn how ETOPS ratings apply to open rotor engines. Any experts can chime in?
I knew I had seen this before growing up as a child, Popular Science, 1985:
https://books.google.com/books?id=rgAAAAAAMBAJ&lpg=PA69&dq=t...
The Antonov An-70 has been in service with "open rotor" engines for 30+ years. It's superior to its western counterparts in every way. i.e. greater speed and payload with less fuel consumption than a C-130 or A400M.
Huh? Only two An-70 prototypes were ever built so it's not really "in service". The early propfan designs, while efficient, were too loud for widespread civil use. Newer open rotor designs are much quieter.
You know what makes the C-130 or the A400M superior? The fact there's more than one operational today.
Then we can duct it.
Everything old is new again... McDonnell Douglas looked into the propfan thing. Boeing looked into the propfan thing. Now it's Airbus' turn. IIRC the technology has been ready for years but the passengers are freaked out by it.
Real problem was noise, not passengers. Immense advances in aeroacoustics over the past 40 years thanks to CFD is the main enabler here.
I think it’s a cool idea but I also know that the nacelles have a safety function of containing the rotor blades in the event of disintegration (e.g. from a bird strike).
If these fans have blades with anywhere near the same kinetic energy, I would be nervous.
Southwest 1380[0] is a case where the cowling didn't quite contain the thrown rotor blade.
They were very lucky that only one person died.
[0] https://en.wikipedia.org/wiki/Southwest_Airlines_Flight_1380
Turboprops have already existed for decades, so perhaps you should already be nervous.
Is this not... a propeller? A turboprop engine?
I’m not an expert but I think the distinction is that the blade tips in these reach supersonic speeds like in turbofans. That is a hard problem to fix because you don’t have the duct to contain the noise and catch the blades if one were to break.
> the distinction is that the blade tips in these reach supersonic speeds like in turbofans
Commercial engines are not designed to have anything to supersonic.
Won’t this be absurdly loud?
I talked with one of the aeroacoustic engineers working on it, she says they expect to match noise levels of current engines.
This is discussed in the article, was there a specific part that was ambiguous?
TFW does say there is an opportunity for reduced noise. However, conventional turboprops are very loud compared to their jet counterparts.
Each revolution of a prop blade sends out a shockwave of air against the airframe. The strength of the shockwave is likely proportional to the instantaneous thrust of the engine, and more blades are likely to weaken or smooth it.
A turbofan has a nacelle to contain the shockwave, and avoid the whole noisy mess.
It's only discussed in a similarly ambiguous way - like that they know noise is a potential problem that they're working on. Though to be fair, the designers probably have no idea themselves, since apparently nobody has built a prototype engine that could be run at the rated thrust level in a way they could check the real-world noise and vibration on.
I would assume that these days you can simulate that increasingly accurately before you need a full-scale prototype.
They could also use active noise cancellation, which is already used in some turboprops like the Q400.
Just an opportunity to sell premium quiet seats at the back, and pleb seats at the front.
With all seriousness, I am thinking whether there are parallels between this proposed plane and the Q400.
I am assuming the target market for this is European short haul flights?
On something like a New York <-> Los Angeles flight I cannot imagine the turboprop beats a 737 in any performance or comfort category.
The article is not about turboprop but about open-rotor engines which are a modern variation of propfan engines.
Russia has also just modernized their IL-114s and got an order from India.
Those are turboprop like the A400m or C-130. The article is about open-rotor engines which are a modern variation of propfan engines.